This invention generally relates to a pump and pumping system, particularly for carbon dioxide, and more particularly to a pump and pumping system for injecting dense phase carbon dioxide into an oil or gas reservoir. This invention can also be used to pump greenhouse gases such as carbon dioxide, methane, nitrous oxide, or chlorofluorocarbons that can exist in a dense phase and dispose of the gases underground or underwater as applicable. In addition, this invention can be used to pump any gas that can exist in a dense phase. Finally, the invention also relates to a method of controlling the pump using a variable speed drive in conjunction with an on-line gas chromatograph to maintain a constant carbon dioxide injection rate.
Contrary to popular belief, the United States still has enormous oil resources. Right now, more than 135 years after the birth of the U.S. oil industry, the nation has twice as much oil remaining in its reservoirs as it has so far produced. For every barrel of oil produced to date, two barrels have been left behind. The U.S. oil industry has produced almost 160 billion barrels, but some 350 billion barrels remain as the target of improved oil technologies. Most of this remaining oil, however, is hard to produce. Locked in complex geologic structures, bypassed by conventional technologies, or simply beyond the capability of today""s recovery processes, this oil remains elusive.
There are a number of enhanced oil recovery systems available for increasing production from oil and gas reservoirs. These include steam injection, carbon dioxide and/or nitrogen gas flooding, waterflooding with chemicals such as polymers, surfactants, and alkalines, and the use of microbes to produce gases or chemicals underground that increase the mobility of remaining oil. One of the more popular enhanced recovery systems in the greater Permian Basin area of West Texas and Southeast New Mexico is carbon dioxide (xe2x80x9cCO2xe2x80x9d) flooding. Carbon dioxide flooding has proven to be among the most promising enhanced oil recovery methods for the United States because it takes advantage of plentiful, naturally-occurring carbon dioxide. When CO2 is injected into a reservoir above its minimum miscibility pressure (a miscible flood), the gas acts as a solvent. The CO2 picks up lighter hydrocarbon components, swelling the total volume of oil and reducing the viscosity of the oil so that it flows more easily. When a field has already been waterflooded, a tertiary CO2 flood will normally provide incremental recovery of about 8% to about 16% of the original oil in place. When CO2 is used instead of waterflood for secondary recovery, the field can produce up to about 40% of the original oil in place.
Usually, CO2 flooding involves the use of CO2 at existing pipeline pressures, and then injecting the CO2 into the field. When existing pipeline pressures are not high enough to inject the CO2 into the reservoir, the CO2 pressure is boosted with a CO2 pump. Existing dense phase CO2 booster pumping technology uses multistage centrifugal or reciprocating pumps with expensive, double mechanical seals in conjunction with high pressure seal oil systems and seal oil cooling systems. This type of prior art pumping system is typically custom built and housed within large support buildings. Consequently, this type of pumping system is costly, uses large amounts of space, is overly complicated, requires considerable maintenance, and is very time consuming to repair or replace.
In addition, those involved in carbon dioxide flooding of a field have attempted to adjust or control CO2 injection rates to account for changes in field conditions and changes in the composition of the CO2 being injected into the field. For example, prior art efforts to control CO2 injection rates include floating the CO2 injection pressure on the pipeline pressure, the CO2 recovery plant pressure, or the CO2 re-injection compressor discharge pressure (for sour gas re-injection where no CO2 recovery plant is available). CO2 injection volumes are controlled using a control valve located at a manifold (for a radial system design) or at the wellhead. The limitation for this type of system is that if the gas composition of the CO2 stream becomes less dense due to a rise in gas stream temperature or gas stream composition, CO2 injection can cease altogether and ultimately lower reservoir processing rates.
In the past, positive displacement plunger pumps have been used to pressurize the dense phase CO2 up to the surface pressure necessary to overcome the lower hydrostatic head caused by the less dense CO2 due to gas impurities and higher temperatures. However, positive displacement pumps are difficult to keep from leaking around the plungers, require much more maintenance than a centrifugal pump, and are not as reliable as centrifugal pumps (run time between maintenance and repair periods are much less than with centrifugal pumps). Where centrifugal pumps were used to boost CO2 injection pressure, the pumps ran at 60 hertz and used a control valve downstream of the pump to adjust the capacity of the pump. This method is inefficient since the medium being pumped is pressurized to a high pressure and is then reduced to a lower pressure across the control valvexe2x80x94resulting in high energy costs per unit volume, reduced reliability of the pump operating away from the best efficiency point on the pump curve, and the initial cost and on-going maintenance of the control valve.
For the foregoing reasons, there is a need for an improved CO2 pump and pumping system and a method of controlling the same.
The present invention is directed to a pump and pumping system for injecting dense phase carbon dioxide into an oil or gas reservoir and for injecting dense phase greenhouse gases such as carbon dioxide, methane, nitrous oxide, or chlorofluorocarbons into a reservoir or underwater.
In one aspect, the invention comprises a downhole electric pump, a motor, and a casing in which the pump and motor are encased or reside, for boosting carbon dioxide or any other dense phase gas. The apparatus has a power source and at least one power lead that connects between the motor and the power source. In another aspect, the invention comprises a method for pumping dense phase gas that involves inserting the casing within an existing piping system by connecting the discharge flange and inlet flange of the piping system with the opposing flanges of the casing. The gas that is pumped by the pump and motor within the casing may be dense phase carbon dioxide gas or any dense phase greenhouse gas.
Among other advantages, the present invention does not require the use of expensive mechanical seals or their associated seal oil systems, does not require large support buildings, and can be maintained, repaired, or replaced easily and inexpensively.
The present invention is also directed to a method of controlling the pump and pumping system using a variable speed drive in conjunction with an on-line gas chromatograph to maintain a constant carbon dioxide injection rate. With such control, the CO2 injection rate into the wellbore will remain constant with varying gas compositions, varying surface or bottom hole temperatures, and varying bottom hole pressures. This will result in optimized CO2 flood sweep efficiency and reservoir processing rates.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily used as a basis for modifying or designing other structures, systems, methods, or algorithms for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.